Synthesis, structure, and fullerene-binding
property of azacalix­[6]­aromatics
were systematically studied. By means of [3 + 3] and [2 + 2 + 2] fragment
coupling protocols, a number of azacalix­[6]­aromatics containing
different combinations of benzene, pyridine, and pyrimidine rings
and various substituents on the bridging nitrogen atoms were synthesized
conveniently in moderate to good yields. The resulting macrocycles
adopt in the solid state symmetric and heavily distorted 1,3,5-alternate
conformations depending on the aromatic building units, whereas, in
solution, they exist as a mixture of conformers that undergo rapid
interchanges relative to the NMR time scale. All macrocycles were
able to form 1:1 complexes with C60 and C70 in
toluene with the association constants up to 7.28 × 104 M–1. In the crystalline state, azacalix­[6]­aromatics
form complexes with C60 and C70 with 2:1, 1:1,
and 1:2 stoichiometric ratios between host and guest. Azacalix­[6]­aromatics
interact with fullerene by forming mainly the sandwich structure in
which C60 or C70 is sandwiched by two macrocycles.
X-ray molecular structures revealed that multiple π–π
and CH−π interactions between concave azacalix­[6]­aromatics
and convex fullerenes C60 and C70 contribute
a joint driving force to the formation of host–guest complexes.